Methods and means for measuring the velocities of localized portions of flowing media

ABSTRACT

The velocity of a localized portion of a fluid medium is measured by focusing a laser beam onto the point to be measured, and then, with a single lens focused on the point, transforming the resulting scattered light as well as the unscattered light into beams parallel to the axis of the lens. The back face of a plate angularly oriented to the axis of the lens reflects one of the beams toward the intersection of an other beam with the front face of the plate. The front face is a half mirror which passes the oncoming other beam and again reflects the already reflected beam. This superimposes the two beams. The composite beam is then detected by a photodetector. Suitable means indicate the resulting heterodyne frequency as a measure of the velocity of the medium.

United States Patent Iten et al.

[451 July 4,1972

[72] Inventors: Paul Dominik lten, Oberrohrdorf; Francois Mottier,Zurich, both of Switzerland [73] Assignee: Brown Boveri 8; CompanyLimited,

Baden, Switzerland [22] Filed: April 22, 1971 21 Appl. No: 136,385

[30] Foreign Application Priority Data April 27, 1970 Switzerland..6247/70 [52] U.S.C1 ..250/218, 331/945, 356/28 [51] Int. Cl. ..G0ln21/26 [58] Field of Search ..250/218; 331/945; 356/28, 356/29 [56]References Cited UNITED STATES PATENTS 3,457,419 7/1969 Rosa ..250/2183,472,593 10/1969 Drinkwater. ..250/218 3,510,665 5/1970 Goolsby.250/218 3,511,227 5/1970 Johnson... .250/218 3,532,427 10/1970 Paine....356/28 3,547,540 12/1970 Shigemoto ..356/28 ZAJ'EE 3,552,855 l/l970Crosswy ..356/28 3,584,956 6/1971 Hines ..356/28 OTHER PUBLICATIONSMeasurement of Localized Flow Velocities With A Laser Doppler Flowmeter,J. W. Johnson Appl. Phys. Letter 8/l5/65,pp. 77-78 Laser DopplerDetection Systems For Gas Velocity Measurement, R. M. Huffaker, AppliedOptics Vol. 9, No. 5, pp. 1026-1039 Primary Examiner.lames W. LawrenceAssistant Examiner-D. C. Nelms Attorney-Toren & McGeady [57] ABSTRACTThe velocity of a localized portion of a fluid medium is measured byfocusing a laser beam onto the point to be measured, and then, with asingle lens focused on the point, transfonning the resulting scatteredlight as well as the unscattered light into beams parallel to the axisof the lens. The back face of a plate angularly oriented to the axis ofthe lens reflects one of the beams toward the intersection of an otherbeam with the front face of the plate. The front face is a half mirrorwhich passes the oncoming other beam and again reflects the alreadyreflected beam. This superimposes the two beams. The composite beam isthen detected by a photodetector. Suitable means indicate the resultingheterodyne frequency as a measure of the velocity of the medium.

7 Claims, 2 Drawing Figures METHODS AND MEANS FOR MEASURING THEVELOCITIES OF LOCALIZED PORTIONS OF FLOWING MEDIA This invention relatesto methods and means for measuring the localized velocities of flowingmedia, and particularly to laser Doppler velocimeters or LDVs which areused for microscopic investigations of velocity fields such as inboundary layer measurements in wind tunnels, or in the movement ofgases, liquids and solid bodies.

Such laser Doppler velocimeters generally include a continuous wavelaser beam focused on the point within the medium at which the velocityis to be measured. A part of the focused beam passes through the mediumand another part is scattered. The moving particles of the medium whichscatter the focused laser light subject the light to a frequency shift.This frequency shift is inversely proportional to the vacuum wave lengthof the laser light. The frequency shift is directly proportional to theindex of refraction of the flowing medium and a component of thevelocity of the scattering particles. This component depends upon themagnitude and the cosine of the direction of the velocity.

To measure this frequency shift optical means superimpose the beamformed by the light which passes unscattered through the medium onto abeam which is formed by light that is scattered by the particles of themedium. The unscattered beam and the scattered beam superimposed thereonare focused jointly on a point of a light detector to form a mixedsignal of comparatively low frequency, namely a heterodyne signal.Depending upon the flow velocity, the frequency of the heterodyne signalhas a value between about 100 Hz and l Gl-lz. Details and embodiments ofsuch a laser Doppler velocimeter are disclosed in the IEEE Journal ofQuantum Electronics, 1966, pages 260-266.

Laser Doppler velocimeters, as compared to classical measuring devicessuch as Pitot tubes and the like, accomplish their beneficial results bymeasuring without mechanically contacting the media in the classicalsense, and thus without disturbing the velocity field. Furthermore, theyare capable of determining the distribution of velocities at variouspoints with high spatial resolution.

However, because of the many degrees of freedom involved, adjustment ofthe optical system in known laser Doppler velocimeters presentsconsiderable difficulties. This is especially so since in order toobtain maximum signal-to-noise ratios for the heterodyne signals, thevectors of the energy flows, that is to say the Poynting vectors, of thereference beam and the signal beam have to coincide.

An object of the present invention is to avoid the above mentioneddisadvantages.

Another object of the invention is to furnish an easily adjustableapparatus of the above mentioned kind.

Still another object of the invention is to improve laser Dopplervelocimeters.

According to a feature of the invention the above disadvantages areobviated and the above objects obtained in a system where a lens focusesa collimated laser beam onto a point in the moving medium, bypositioning a second lens so that its focal plane intersects the focalplane of the first lens at the measuring point. The lens transformslight scattered from the focus by the moving medium and the unscatteredlight which passes through the moving medium into light bundles, rays,or beams which are recollimated and extend parallel to the optical axisof the second lens. Optical means, after superimposing a beam ofunscattered light on a beam of scattered light so the two follow thesame path and form a single composite beam, then produce a heterodynesignal from the composite beam. This signal is a measure of the velocityof the moving medium at the point at which it is measured. That is tosay it is a measure of the localized or local velocity of the medium.

According to another feature of the invention the optical means includesa transparent parallel-faced plate in the path of the beams. The facesor face planes of the plate are inclined relative to the optical axis ofthe second lens. Reflective surfaces of these planes superimpose thebeam composed of unscattered light upon the beam of scattered light. Forsimplicity, a beam, bundle or ray of unscattered light is herein calledan unscattered light beam or an unscattered beam. Similarly, a beam,bundle or ray of scattered light is called a scattered light beam orscattered beam.

According to yet another feature of the invention the plane of the platefurther from the second lens and in the path of a beam of unscatteredlight reflects internal rays. On the basis of its inclination relativeto the axis of the second lens it directs the unscattered beam throughthe plate toward the intersection of a scattered light beam with theplane closest to the lens. At the intersection, an uncoated surfacereflects a small fraction of the unscattered beam into the plate andrefracts the intersecting scattered beam. The beams leave theintersection in the same direction. In this way a scattered beam and anun scattered beam are superimposed on each other to form a compositebeam.

According to yet another feature of the invention the optical meansincludes an iris, diaphragm, or mask having an aperture located behindthe plate. The aperture permits passage only of the superimposed mixedbeam or bundle composed of the collinear scattered and unscatteredbeams. A third lens then focuses the composite beam on the aperture of asecond iris. A light detector measures the intensity of the light whichenters the second aperture.

By virtue of these features a few adjustments of the optical system aresufficient to obtain the desired result. The most important of theseadjustments is to set the first two lenses so their focal planesintersect the point within the medium at which the velocity is to bemeasured. Altemately the first two lenses may be adjusted so their focalplanes coincide. When the focal planes intersect at the point ofmeasurement or coincide with each other, projection of collimated lightonto the first lens causes the scattered light beam and the unscatteredlight beam to be recollimated and parallel to each other after they passthrough the second lens.

According to another feature of the invention the optics are arranged sothat they use scattered light which, relative to the optical axis of thesecond, leaves the medium symmetrical to the unscattered light orpartial bundle or beam. By virtue of this feature, and the symmetricalconstruction resulting therefrom, wave front distortions by the imageforming means are substantially obviated. With the optics arranged touse the symmetrically oriented scattered light, the second lens, whichtransforms the rays of the scattered light into bundles or beams runningparallel to its optical axis, effectively serves to form images onlywith portions lying symmetrical to its center. For reasons relating tothe production of lenses these symmetrically located portions haveidentical optical characteristics. Thus they also exhibit the same imageforming errors, such as wave front distortions due to sphericabberations. Thus, if both partial bundles or beams are distorted orotherwise changed as they pass through the lens, both of the bundles orbeams are subjected to identical distortions or changes. Thus, imageforming errors have no influence upon the measuring results.

According to yet another feature of the invention an intensityattenuator is positioned in the path of the unscattered beam or partialbundle as it passes from the second lens.

According to another feature of the invention the attenuator includes anadjustable gray filter. The latter adjusts the optimum condition betweenthe intensity of the scattered light beam or bundle and the unscatteredlight beam.

According to still another feature of the invention suitable meansadjust the inclination of the plate and its planes relative to the axisof the second lens.

According to still another feature of the invention the plate, the firstiris, the third lens, the second iris, and the light detector aregrouped to form a mechanical unit. Mechanical means responding to theinclination of the planes of the plate toward the optical axis of thesecond lens displace the first iris and its aperture, the third lens,the second iris and its aperture, and the light detector in a directionperpendicular to the axis so that the composite beam always passesthrough the aperture of the first iris and the second one regardless ofthe inclination of the planes on the plate. This arrangementsignificantly simplifies adjustment of the optical system.

These and other features of the invention are pointed out in the claims.Other objects and advantages of the invention will become obvious fromthe following detailed description when read in light of theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a block diagram illustratingan apparatus embodying features of the invention; and

FIG. 2 is a block diagram illustrating details of a portion of theapparatus in FIG. 1.

In FIG. 1, a medium 1, whose velocity is to be measured at specifiedlocations within the medium, flows in the direction x. A lens 2 focusesthe coherent light emerging from a continuous wave laser 3 onto thepoint or location to be measured within the medium 1 so as to illuminatethat point. As a result of the illumination unscattered light in theform of a partial bundle or beam 4 exits from the medium 1 toward a lens5. A portion of the light is scattered in a number of directions. Thatportion of the scattered light which extends symmetrically to theunscattered light about the optical axis 2 of the lens 5 is identifiedas a scattered bundle or partial bundle, or ray, or beam 6. This beam 6also impinges on the lens 5. The lens 5 is adjusted so that its focalplane intersects the point at which the velocity is to be measured inthe medium 1. More specifically, as shown in FIG. I, the focal planes ofthe lens 2 and the lens 5 coincide with each other.

Both the unscattered light beam and the scattered light beam arerecollimated by the lens 5. They leave the lens 5 parallel to the axisof the lens 5. The paths of these beams are symmetrical about the axisz. These two beams impinge upon a plate 7 having parallel faces. Theplate is arranged to rotate within the x-z plane about an axisperpendicular to the plane of the drawing. The plate superimposes thebeam 4 upon the beam 6.

In order to perform this superimposition the plate 7 functions as a beamsplitter. On its rear face opposite to the impinging light the plate 7is partially mirrored. Specifically the lower portion of it is mirrored.The beam 4 is refracted by the plate 7, and as it impinges upon the rearface is reflected upwardly. The angle of the plate 7 about the axisperpendicular to the plane of the drawing is such that the interiorlyreflected beam 4 intersects the front face of the plate 7 at theintersection of that front face with the beam 6. At the point ofintersection the plate 7 is not mirrored to allow passage of the beam 6past the face and into the plate and at the same time to reflect a smallfraction of the beam 4 back into the interior of the plate. There is nocoating necessary because any glass/airboundary or air/glass-boundaryreflects at least 4 percent of the incident power (see for example F.W.Sears, Optics, page 174, Addison-Wesley Publishing Company, Inc.,Reading, Mass). The angle of the plate 7 is such that the beam 4 andbeam 6 are now superimposed on each other and travel the same paththrough the plate 7 to the back face where they are again refracted as acomposite beam 9.

An adjustable gray filter 8 between the lens 5 and the plate 7 adjuststhe ratio of the optical intensities of the scattered beam 6 and theunscattered beam 4.

The composite beam 9 which exits from the plate 7 impinges upon anaperture 10 in a suitable iris. The aperture 10 cuts out all those rayswhich are not designated for further processing. Its diameter isapproximately one millimeter and corresponds to the diameter of thecomposite beam. A control lens 11 focuses the light which leaves theaperture 10 onto the aperture 12 of a second iris. The lens 1 1 forms animage corresponding to the points to be measured in the medium 1 at theaperture 12. The diameter of the aperture 12 thus determines the size ofthe area or point in the medium to be measured. The diameter of theaperture 12 is preferably of a magnitude of approximately 10am.

A photomultiplier 13 measures the intensity of the light passing throughthe aperture 12 and converts it into an electrical potential. Afrequency analyzer 14 then analyzes the frequency for the purpose ofdetermining the speed of the moving medium at the portion to bemeasured. Photomultipliers such as 13 and frequency analyzers such as 14as well as other devices for performing their functions in thisenvironment are well known. For example, the beforementioned article inthe IEEE Journal of Quantum Electronics, Vol. QE-2 No. 8 of August 1966,pages 260-266 describes a photomultiplier and a corresponding readoutsystem for the purpose of measuring the velocities of flowing media.

The quality of velocity measurements in this environment is determinedby the adjustment of the various members. The focal plane of the firstlens 2 is adjusted so as to intersect the point to be measured withinthe medium 1. The second lens 5 is adjusted so its focal plane alsointersects this point. On the other hand it may be adjusted so as tocoincide with the focal plane of the first lens 2. The latter occurs,for example, if the optical axis of the two lenses 2 and 5 coincide.

A second adjustment operation resides in turning the plate 7 about apivot in the x-z plane. This pivot lies along an axis perpendicular tothe plane of the drawing. This adjustment determines the direction ofthe desired scattered light beam leaving the medium 1. Subsequently theiris with the aperture 10, the lens 11 and the iris with the aperture 12as well as the photomultiplier 13 must be moved parallel to the z axis.

The last mentioned adjustment, namely turning of the plate 7 andparallel displacement of the iris with the aperture 10, the lens 11, theiris with the aperture 12, and the photomultiplier 13 is simplified bybuilding the parts together into a unit as shown by the dotted line inFIG. 1. Within that unit, means are provided which respond to turning ofthe plate 7 within the xz plane and about a pivot perpendicular to theplane of the drawing by displacing the iris with the aperture 10, thelens 11, the iris with the aperture 12, and the photomultiplier 13,parallel to each other simultaneously. The setting member for the platemay then be calibrated directly into degrees of angles which correspondto the angle at which the scattered light beam to be measured leaves themedium 1.

FIG. 2 illustrates the apparatus for simultaneous movement of the plate7 and the irises as well as the lens 11 and the photomultiplier 13. Herea knob 20 turns the plate 7 about a pivot 21. While turning the plate 7,the knob 20 also turns a cam 22 so that a cam follower 23 moves a bar 24that simultaneously shifts the irises as well as the lens 11 and thephotomultiplier 13 simultaneously up and down. Other systems such asservo-mechanisms may be used for causing cooperation and coactionbetween the plate 7 and the remainder of the elements within the box soas to provide a path for the beam 9 from the plate 7 to thephotomultiplier 13.

The term iris as used herein is used in the sense of any diaphragmhaving an opening whether it be adjustable or not. The apertures 10 and12 formed by the irises may or may not be adjustable depending upon theparticular needs of the measuring system.

The velocities to be measured at any particular point or localized areaare referred to herein and in the claims as the local velocities. Itwill be understood by this term that it refers to the velocity of themedium at any particular point at which the velocity is measured.

It will be noted that the tilted plate 7 has the effect of reflectingthe beam 4 twice and subjecting it twice to refraction. In this way thebeam 4 is shifted parallel to its original path by a distance determinedby the angle of the plate 7. The angle of the plate 7 is such as toshift the beam 4 so that it coincides with the beam 6 which the plate 7subjects twice to refraction. The beam 6 is one of many scattered beamswhich propagates parallel to the beam 4. It is the beam that left thepoint being measured symmetrically with the beam 4 relative to theaxis 1. It thus is equidistant with the beam 4 relative to the axis z.In this way one measures the x-component of the velocity which is theonly velocity component in a cylindrical tube.

While an embodiment of the invention has been described in detail, itwill be obvious to those skilled in the art that the invention may beembodied otherwise without departing from its spirit and scope.

What is claimed is:

1. An apparatus for measuring local velocities of a flowing medium,comprising first lens means for focusing a laser beam into the mediumonto a point at which the velocity is to be measured so that a part ofthe light of the laser beam passes unscattered through the medium and apart of the light of the laser beam is scattered, an optical means forsuperimposing the unscattered light onto a portion of the scatteredlight from the focus; said optical means including second lens means inthe path of the scattered and the unscattered light and having a focalplane which intersects the focal plane of the first lens means at thepoint at which the velocity is to be measured for transforming a portionof the scattered light and a portion of the unscattered light into ascattered light beam and an unscattered light beam which extend parallelto the optical axis of said second lens means, beam splitter meansincluding an angularly adjustable reflecting plate in the path of saidlight beams for superimposing one of said light beams onto the other ofsaid light beams and forming a composite beam which defines a heterodynefrequency, an electrical light sensitive means responding to thecomposite beam for producing an indication corresponding to theheterodyne frequency.

2. An apparatus as in claim 1, wherein said light sensitive meanscomprises first aperture forming means in the path of the light emergingfrom said beam splitter means for passing only the composite beam, thirdlens means located in the path of the composite beam emerging from saidfirst aperture forming means for focusing the composite beam, secondaperture forming means for forming an aperture at the focus of saidthird lens means, and a light detector in the path of the composite beamemerging from said second aperture forming means for measuring theintensity of the light passed through said second aperture formingmeans.

3. An apparatus as in claim 1, wherein said optical plate in said beamsplitter means includes a first planar face and a second planar faceparallel to the first planar face, said planar faces being angularrelative to the axis of said second lens means, said first planar facebeing closer to said first lens means than said second planar face,reflecting means on said second planar face in the path of one of thelight beams for interiorly reflecting the one of the light beams towardsaid first planar face, said first planar face being in the path of theother of the light beams, the angle of said faces being inclinedrelative to the axis of said second lens means so that said reflectiveportion of said second face reflects the one of said light beams to theintersection of the other of said light beams with said first planarface, said second planar face at the intersection of said beams beingpartially reflective and partially non-reflective so that the one ofsaid light beams is reflected interiorly of the plate in one directionand the other of the light beams is refracted by the plate in the samedirection.

4. An apparatus as in claim 3, wherein the one of said light beams isthe unscattered light beam and the other of said light beams is thescattered light beam.

5. An apparatus as in claim 1, wherein said optical means includes anoptical attenuator positioned in the path of the light beam between saidsecond lens means and said beam splitter means.

6. An apparatus as in claim 5, wherein said attenuator means ispositioned in the path of the unscattered light beam.

7. An apparatus as in claim 3, wherein said light sensitive meanscomprises pivot means for changing the angle of said plate relative tothe axis of said second lens means, said light sensitive means furtherincluding a light detector in the path of said composite beam, movingmeans coupled to said pivot means for moving said light detector meansinto the path of said composite beam in response to the movement of theangle of said plate.

* )IK 'l TJNTTED STATES PATENT oTTTcE (JERTEHQATE F QEQTWN Patent No. 375 OZ 9 Dat d July 4 1972 I Paul Dominik Iten and Francois Mottier It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In claim 3, .Line 1, the term "optical plate" should read --ref1ectingplate-- Signed and sealed this 27th day of November 1973.

(SEAL) Attest:

RENE D. TEGTMEY'ER EDWARD M.PLETCHER,JR. K

Acting COTIHTLISSIO116T of Patents Attesting Officer FORM PO-1050(10-69) USCOMM DC eomepeg 1* us. covzrmmzm PRINTING OFFICE: waso-ase-aaa.

UNTIED STATES PATENT OFFICE CERTIFICATE OF CORRECTEON Patent No. 3 ,6-75,029 Dated July 4 1972 lnventofls) Paul Dominik Iten and Erancois Mottier It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

In Claim. 3, line 1, the term "optical plate" should read --reflectingplate-- Signed and sealed this 27th day of November 1973.

(SEAL) Attest:

EDWARD M.PLETCHER,JR. RENE D. TEG'ljMEYER Attesting Officer ActingCommissioner of Pate t FORM Do-1050 0-69) v USCOMM-DC 60376-P69 U.$,GOVERNMENT HUNTING OFFICE [9C9 0-366-334,

1. An apparatus for measuring local velocities of a flowing medium,comprising first lens means for focusing a laser beam into the mediumonto a point at which the velocity is to be measured so that a part ofthe light of the laser beam passes unscattered through the medium and apart of the light of the laser beam is scattered, an optical means forsuperimposing the unscattered light onto a portion of the scatteredlight from the focus; said optical means including second lens means inthe path of the scattered and the unscattered light and having a focalplane which intersects the focal plane of the first lens means at thepoint at which the velocity is to be measured for transforming a portionof the scattered light and a poRtion of the unscattered light into ascattered light beam and an unscattered light beam which extend parallelto the optical axis of said second lens means, beam splitter meansincluding an angularly adjustable reflecting plate in the path of saidlight beams for superimposing one of said light beams onto the other ofsaid light beams and forming a composite beam which defines a heterodynefrequency, an electrical light sensitive means responding to thecomposite beam for producing an indication corresponding to theheterodyne frequency.
 2. An apparatus as in claim 1, wherein said lightsensitive means comprises first aperture forming means in the path ofthe light emerging from said beam splitter means for passing only thecomposite beam, third lens means located in the path of the compositebeam emerging from said first aperture forming means for focusing thecomposite beam, second aperture forming means for forming an aperture atthe focus of said third lens means, and a light detector in the path ofthe composite beam emerging from said second aperture forming means formeasuring the intensity of the light passed through said second apertureforming means.
 3. An apparatus as in claim 1, wherein said optical platein said beam splitter means includes a first planar face and a secondplanar face parallel to the first planar face, said planar faces beingangular relative to the axis of said second lens means, said firstplanar face being closer to said first lens means than said secondplanar face, reflecting means on said second planar face in the path ofone of the light beams for interiorly reflecting the one of the lightbeams toward said first planar face, said first planar face being in thepath of the other of the light beams, the angle of said faces beinginclined relative to the axis of said second lens means so that saidreflective portion of said second face reflects the one of said lightbeams to the intersection of the other of said light beams with saidfirst planar face, said second planar face at the intersection of saidbeams being partially reflective and partially non-reflective so thatthe one of said light beams is reflected interiorly of the plate in onedirection and the other of the light beams is refracted by the plate inthe same direction.
 4. An apparatus as in claim 3, wherein the one ofsaid light beams is the unscattered light beam and the other of saidlight beams is the scattered light beam.
 5. An apparatus as in claim 1,wherein said optical means includes an optical attenuator positioned inthe path of the light beam between said second lens means and said beamsplitter means.
 6. An apparatus as in claim 5, wherein said attenuatormeans is positioned in the path of the unscattered light beam.
 7. Anapparatus as in claim 3, wherein said light sensitive means comprisespivot means for changing the angle of said plate relative to the axis ofsaid second lens means, said light sensitive means further including alight detector in the path of said composite beam, and moving meanscoupled to said pivot means for moving said light detector means intothe path of said composite beam in response to the movement of the angleof said plate.